As shown in FIG. 5, the conventional back mirror comprises a base plate 50 and a mirror housing 60 provided in a detachable manner. A permanent magnet 52 is buried on an assembly plane 51 of the base plate 50 and a metal piece 62 is buried on an assembly plane 61 of the mirror housing 60 at a position corresponding to the position of the permanent magnet 52 so as to be coupled by means of magnetic attraction. Through-holes 53, 63 respectively are provided at the centers of the assembly planes 51, 61 of the base plate 50 and the mirror housing 60, with the through-hole 53 being smaller than the through-hole 63. A pin slot 54 is provided at a side of the through-hole 53; and a drop preventing wire 70 having a stopper at the end thereof is provided in such a manner that it should be able to freely pass through the through-hole formed at the leading end of a mirror supporting plate. The stopper of the drop preventing wire 70 should prevent the dropping of the mirror housing 60.
The conventional back mirror as described above has the advantages of ease of assembly and increased productivity and joining the base plate and the mirror housing, is simplified and a saving of the manufacturing cost is realized owing to the decreased number of components.
However, the absorption of the impacts and vibrations during a running is done only by means of magnetic forces of a permanent magnet, and therefore, there is the possibility that the mirror housing which is coupled with the base plate can be displaced relative to the latter. Consequently, the posture of the mirror housing is deviated from the driver's observing angle, with the result that the back mirror loses the intended function.
Particularly, when the impacts and vibrations of the car body are severe, the wire serves as the only means for preventing the dropping of the mirror housing, and therefore, the supporting structure is not strong enough. Further, in the case where the mirror housing is to be recoupled with the base plate after a detachment, there is no physical criteria for the coupling, and therefore, too much attention is demanded to the driver. In such a case, the recoupling is apt to be done in an incorrect manner, and therefore, there are usually required repetitions of the coupling procedure.